fix: return 0 for constant input in incr/skewness and incr/kurtosis

[apoorva-01]

Resolves #287.

• Fixes @stdlib/stats/incr/skewness and @stdlib/stats/incr/kurtosis returning NaN instead of 0 when every value is equal.

The accumulators divide by a power of the second central moment M2. For constant input M2 is exactly 0, so it's a 0/0 division (NaN). The expected value for a constant data set is 0 (same as the pandas reference in the issue). The fix guards a zero M2 in both accumulators, in the running-update and no-argument query paths. These two are the only stats/incr packages doing this division; incr/nanskewness wraps skewness and inherits it, incr/summary still passes.

Related Issues

• Incremental skewness and other higher moments when provided equal values do not yield expected results #287

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